A need presently exists for an electrically-conductive additive for paints, plastics, papers, and similar products which (a) will provide desirable electrostatic discharge and electromagnetic shielding properties, (b) will provide long service life, (c) is inexpensive to make and use, and (d) will enable the achievement of desired colors and/or transparency.
Carbon black, various metals, certain organic amines and amides, and doped tin oxide have been used heretofore as additives in paints, plastics, and paper products to provide desirable electrostatic discharge and/or electromagnetic shielding properties. However, these additives have significant shortcomings. Carbon black and the various metal additives used heretofore generally hinder and/or prevent the attainment of certain desirable colors and/or transparency. Products containing carbon black are also susceptible to sloughing. Organic amine and organic amide additives, on the other hand, generally have undesirably high solubilities and volatilities. Thus, products containing amine or amide additives typically have short service lives, low durabilities, and poor weather and humidity resistance characteristics. Finally, although doped tin oxide is desirably light-colored and electrically conductive, doped tin oxide is costly to produce.
Zinc oxide is a wide band gap semiconductor which can be made conductive by doping with Group IIIA metal oxides. For example, U.S. Pat. No. 4,990,286 discloses a chemical vapor deposition (CVD) process for coating a substrate surface with a transparent, electrically-conductive, zinc oxyfluoride film. The composition of this zinc oxyfluoride film can generally be represented by the formula ZnO.sub.1-x F.sub.x wherein x is a value in the range of from 0.001 to 0.1. The CVD process of U.S. Pat. No. 4,990,286 can be used to produce liquid crystal display devices, solar cells, electrochromic absorbers and reflectors, energy conserving heat mirrors, and antistatic coatings.
In the CVD process of U.S. Pat. No. 4,990,286, the substrate in question is heated and then contacted with a vapor mixture including: a volatile and highly reactive zinc-containing compound, a volatile oxygen-containing compound, and a volatile fluorine-containing compound. The temperature of the heated substrate must be sufficient to induce, for the particular reactants chosen, a film forming deposition reaction. Oxidation occurs and the zinc oxyfluoride film is deposited only as the vapor mixture contacts the heated substrate.
Volatile zinc-containing compounds suitable for use in the CVD process of U.S. Pat. No. 4,990,286 include diethyl zinc and dimethyl zinc. When, for example, the reactants used in the CVD process are diethyl zinc, ethyl alcohol, and hexafluoropropylene, the substrate must be heated to a temperature in the range of from about 350.degree. C. to about 500.degree. C. If the particular substrate in question cannot withstand such temperatures, alternative reaction mixtures having lower film deposition reaction temperatures can be used.
Unfortunately, conductive zinc oxyfluoride films such as those produced by the CVD process of U.S. Pat. No. 4,990,286 are susceptible to sloughing, scratching, and abrasion. Further, due to the difficulty and expense involved in coating large items and items having numerous and/or intricate surfaces, CVD processes do not provide a practical means for producing conductive plastic articles. Moreover, zinc oxyfluoride films of the type produced by the CVD process of U.S. Pat. No. 4,990,286 are not obtained in, and cannot be readily converted to, particulate forms which are suitable for addition to paints, plastics, papers, and other such products.
Other factors also mitigate against the commercial use of CVD-type processes. Due to the highly reactive nature of the zinc-containing compounds required for use in CVD-type processes, the use of such processes on a commercial scale would be quite dangerous. The use of CVD-type processes on a commercial scale would also not be cost effective since (1) the volatile zinc-containing compounds used in these processes are expensive and (2) only a fraction of the zinc-containing compound used in a CVD-type process actually reacts and deposits on the substrate. The requirement that heated substrates be used also reduces the commercial viability of CVD-type processes.
As is well known in the art, non-doped particulate zinc oxides have been produced using vapor phase oxidation processes. Vapor phase oxidation processes used for producing particulate zinc oxide products are generally discussed, for example, in Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, John Wiley and Sons, 1978, Volume 24, the entire disclosure of which is incorporated herein by reference.
U.S. Pat. No. 3,551,356 discloses a process for coating a particulate zinc oxide material in order to increase its electrical conductivity. In one embodiment of the process, a reportedly stable and conductive particulate zinc oxide product is produced by heating particulate zinc oxide in a non-oxidizing (preferably nitrogen) atmosphere in the presence of both (1) a fluorine compound and (2) zinc metal vapor. During the heating process, the particulate zinc oxide is preferably maintained at a temperature in the range of from about 600.degree. C. to about 950.degree. C. for a period of about 20 minutes. It is also preferred that moist nitrogen be circulated through the process system during the initial stage of the heating process (i.e., during no more than the first five minutes of the heating period) and that dry nitrogen be circulated through the process system during the remainder of the heating period.
The coating process disclosed in U.S. Pat. No. 3,551,356 and the product produced thereby have numerous undesirable characteristics. The particulate zinc oxide product produced in accordance with U.S. Pat. No. 3,551,356 is only surface coated. Thus, substantial conductivity losses are realized as surface attrition of the particulate product occurs. Additionally, at least two entirely separate processes are required to complete the coating method of U.S. Pat. No. 3,551,356. In the first process, a particulate zinc oxide product is produced and recovered. In the second process, the particulate zinc oxide material is fluorine treated and heat treated. Further, in comparison to the inventive process described hereinbelow, the coating process of U.S. Pat. No. 3,551,356 is very slow.